1. Field of the Invention
The present invention relates to a properly biased antiparallel (AP) pinned spin valve sensor with a metallic pinning layer and no read gap offset and, more particularly, to such a spin valve sensor wherein a net demagnetizing field HD and a net sense current field HI acting on a free layer of the sensor is counterbalanced by a ferromagnetic coupling field HF and a biasing field HB acting on the free layer.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfacing of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The sensitivity of the spin valve sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. In a spin valve sensor subjected to positive and negative magnetic signal fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the resistance of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing equal positive and negative fields from the magnetic disk. Accordingly, the bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. The location of the bias point on the transfer curve is typically influenced by four major forces on the free layer, namely a ferromagnetic coupling field HF between the pinned layer and the free layer, a net demagnetizing field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer and a net image current field HIM from the first and second shield layers.
A gap offset is employed for obtaining the net image sense current field on the free layer from the first and second shield layers. A gap offset is where the free layer is located closer to one of the shield layers, typically the second shield layer, than the other shield layer, typically the first shield layer. The image current field from a shield layer is due to an image current in the shield layer which, in turn, is caused by the sense current flowing through the spin valve sensor. With increasing linear bit density read heads, a gap offset becomes impractical because of the risk of shorting between first and second lead layers and one of the shield layers, typically the second shield layer. Accordingly, the free layer should be centered (no offset) between the first and second shield layers in order to promote linear bit density. In such a read head the net image current field is not available for properly biasing the free layer.
An antiparallel pinned (AP) spin valve sensor is employed for reducing the aforementioned demagnetizing field HD. In contrast to a single pinned layer an AP pinned layer structure has a nonmagnetic spacer layer which is located between ferromagnetic first and second AP pinned layers. The first AP pinned layer, which may comprise several ferromagnetic thin films, is exchange-coupled to the pinning layer, with its magnetic moment pinned by the pinning layer in a first direction. The magnetic moment of the second AP pinned layer is pinned in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer. The magnetic moments of the first and second AP pinned layers subtractively combine to provide the AP pinned layer structure with a net magnetic moment. The direction of the net magnetic moment is determined by the thicker of the first and second AP pinned layers. The thicknesses of the first and second AP pinned layers are chosen so that the net magnetic moment is small. A small net magnetic moment equates to a small demagnetizing (demag) field exerted on the free layer by the AP pinned layer. Since the antiferromagnetic exchange coupling between the first AP pinned layer and the pinning layer is inversely proportional to the net moment, this results in a large exchange coupling between the first AP pinned layer and the pinning layer.
It is further desirable to employ a metallic material for the pinning layer instead of nickel oxide (NiO) since a metallic pinning layer can be thinner. A thinner pinning layer promotes the aforementioned linear bit density. However, a metallic pinning layer increases the net sense current field on the free layer which must be counterbalanced by other fields acting on the free layer.
Accordingly, there is a need for properly biasing the free layer when the free layer is centered between the first and second shield layers (no read gap offset), when a metallic pinning layer is employed for promoting linear bit density and when an AP pinned layer structure is employed instead of a single pinned layer for reducing the net demagnetizing field on the free layer and increasing the exchange coupling field between the first AP pinned layer and the pinning layer.
The present invention provides an AP pinned spin valve sensor which has a metallic pinning layer and which requires no read gap offset in order to properly bias a free layer of the spin valve sensor. The free layer is properly biased by a net demagnetizing field HD and a net sense current field HI acting on the free layer which are counterbalanced by a ferromagnetic coupling field HF and a biasing field HB acting on the free layer. The biasing field is accomplished by a ferromagnetic biasing layer which is separated from the free layer by a nonmagnetic isolation layer. Depending upon the direction of the sense current through the sensor, the ferromagnetic coupling field HF is either positive or negative in order to support the biasing field HB. Regardless of the direction of the sense current HD+HI=HF+HB. The biasing layer has a high resistance and is preferably composed of cobalt iron niobium (CoFeNb) or cobalt iron niobium hafnium (CoFeNbHf). With this arrangement sense current shunting through the biasing layer is reduced. The pinned layer structure is an AP pinned layer structure which reduces the net demag HD on the free layer but, more importantly, which increases the exchange coupling field between the pinning layer and the AP pinned layer structure. Accordingly, the pinning of the magnetic moment of the AP pinned layer structure is increased when the read head is subjected to unwanted thermal transients. The preferred pinning layer is platinum manganese (PtMn) which permits the ferromagnetic coupling field to be either positive or negative, which is thinner than a nickel oxide (NiO) pinning layer and which supports the sense current field HI acting on the free layer.
An object of the present invention is to provide a spin valve sensor wherein a net demagnetizing field of an AP pinned layer structure and a net sense current field of all conductive layers other than the free layer is counterbalanced by a net ferromagnetic coupling field and a net biasing field acting on the free layer.
Another object is to provide the foregoing sensor wherein a pinning layer of the sensor is metal and a biasing layer of the sensor has a high resistance as compared to the other ferromagnetic layers of the sensor.
A further object is to provide the foregoing sensors with a sense current field which supports a pinning action between the pinning layer and the AP pinned layer of the sensor.
Still another object is to provide the foregoing sensor with a platinum manganese pinning layer and a copper spacer layer which can be provided with one thickness for providing a positive ferromagnetic coupling field and another thickness for providing a negative ferromagnetic coupling field.
Still a further object is to provide a method for making any of the foregoing spin valve sensors.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.